When subjected to some anti-plane shear mode III loading, segmentation of the crack front frequently occurs during propagation: even if the crack is initially planar, propagation produces facets/segments rotated toward the shear free direction. Here, we examine, both experimentally and theoretically, the effect of this microstructure on the effective macroscale brittle fracture toughness. Experiments performed on PMMA beams reveal that the critical load leading to abrupt rupture increases with mode III to mode I ratio. This apparent macroscopic toughening is usually taken into account by invoking a specific mode III toughness in addition to the mode I one. By applying thoroughfully a micro/macroscale Cohesive Zone (CZ) model that we have recently developed, we demonstrate that an additional material constant is useless here since this toughness increase can be attributed mainly to the presence of the facets at the microscale, whose geometry can be anticipated to depend on the classical mode I material constants. More precisely, two related physical mechanisms are generated due to the formation of a disconnected crack front: (i) changes in fractured surface area in comparison to a straight propagation, and (ii) crack shielding caused by the facets that reduce the effective crack opening. While the first effect is obvious to quantify, we show that the second plays an essential role but is more complex to take into account: it depends on the solution of the three-dimensional elasticity problem in presence of the facets, that is considered in the CZ model. We illustrate on the experiments how to use this approach in practice to determine the critical fracture threshold.

当受到一些反平面剪切模式 III 载荷时，裂纹前沿的分段在扩展过程中经常发生：即使裂纹最初是平面的，扩展也会产生向无剪切方向旋转的小面/分段。在这里，我们从实验和理论上研究了这种微观结构对有效宏观脆性断裂韧性的影响。在 PMMA 梁上进行的实验表明，导致突然断裂的临界载荷随着模式 III 与模式 I 的比率而增加。除了模式 I 韧性之外，通常通过调用特定模式 III 韧性来考虑这种明显的宏观增韧。通过彻底应用我们最近开发的微观/宏观凝聚区 (CZ) 模型，我们证明了额外的材料常数在这里是无用的，因为这种韧性的增加主要归因于微观尺度小面的存在，其几何形状可以预期取决于经典模式 I 材料常数。更准确地说，由于不连续裂纹前沿的形成，产生了两种相关的物理机制：（i）与直线传播相比断裂表面积的变化，以及（ii）由减少有效裂纹张开的小面引起的裂纹屏蔽. 虽然第一个效应很明显可以量化，但我们表明第二个效应起着重要作用，但考虑起来更复杂：它取决于存在小平面的三维弹性问题的解决方案，这在CZ 模型。